The present invention relates to the field of so-called low dimensional semiconductor devices. More specifically, the present invention relates to semiconductor devices which use so-called quantum dots to either store charge or detect incident radiation. The devices can be used for a range of applications, but are particularly intended for use as optically activated devices such as optical detectors or memory structures.
The electrons and holes in an ideal bulk semiconductor have a continuous spectrum of energy states. Confinement of the carriers in one or more dimensions modifies this energy spectrum by a quantisation of the k-vector along the confinement direction(s). In a quantum dot, the motion of the carriers is restricted in all three spatial dimensions. Consequently, the energy spectrum of the dots consists of a series of discrete levels. As the size of the quantum dot reduces, the energy spacing between these discrete levels increases. The maximum number of electrons which can occupy each electron level is two, corresponding to the up and down spin states. Similarly, each hole level has an occupancy of two. Optical transitions occur between the discrete electron and discrete hole levels.
There is a need for an optical detector which is capable of detecting a single photon. Recently, this need has been heightened by the advent of quantum cryptography of optical signals. In essence, quantum cryptography relies upon the transmission of data bits as single particles in this case photons, which are indivisible. One way in which the data can be encoded is via the polarisation of the electric field vector of the photons. The key component of such a system is a detector which can respond to individual photons. It has been proposed that quantum cryptography can be used to transmit the key for the encryption of data. Examples of information which might be encrypted in this way are internet data or data from automatic teller machines.
Single photon detection is also useful as a low level light detection means for spectroscopy, medical imaging or astronomy. An optimum signal to noise ratio is achieved when a single photon is detected, as the noise is then limited by the source and it is completely independent of noise arising due to the amplifier or detector itself.
A single photon detector could also be used for time-of-flight ranging experiments where the distance of an object from a fixed point is measured by calculating the time over which a single photon takes to return to a detector. This technique can also be used to scan the surface of an object, even a distant object, to form a spatial image of its surface depth, thickness etc.
Single photon detectors are available in the form of photo multiplier tubes (PMT) and single photon avalanche photo diodes (SPAD). PMTs have the disadvantage of having low quantum efficiency, being expensive, bulky, mechanically fragile, requiring high biasing voltages and cooling. They can also be damaged and can require a long settling time after exposure to high light levels or stay magnetic fields. On the other hand, SPADs have the disadvantage of having a relatively low gain and high dark count rates, especially when operated at higher repetition rates. They are also expensive and require high bias voltages and external cooling.
Applications for memory devices are well known. Memory devices which use quantum dots are also know, for example, Imamura et al. Jpn. J. Appl. Phys. Vol. 34 (1995) L1445-L1447. Here, the device has a plurality of InAs quantum dot of different sizes.
Upon illumination, an electron-hole pair is excited in the quantum dot. Due to the biasing of the structure, the electron is swept vertically down through the structure into an ohmic metal contact. The hole is trapped in the quantum dot Due to the movement of electrons, a photocurrent flows. Only a finite number of holes can be stored in a dot. If this finite number is reached, no further holes can be stored in the dots and hence no photocurrent due to dissociated electrons can flow. Thus, stored charge can by detected by a lack of photocurrent.
The carrier trapping properties of quantum dots is also illustrated in Yusa et al Appl. Phys. Lett. 70 (1997) 345. Here, a plurality of quantum dots are used to show trapping effects which occur when a two dimensional electron gas (2DEG) is illuminated.
The device of the present invention can be primarily configured as an optical detector which is capable of detecting single photons or an optical memory. Other device configurations are also possible.
In a first aspect, the present invention provides a semiconductor device comprising first and second active layers separated by a first barrier layer, means for applying electric field normal to the first and second active layers and detecting means for detecting a change in a characteristic of the first active layer, wherein the first active layer is a quantum well layer capable of supporting a two dimensional carrier gas and the second active layer comprises a plurality of quantum dots.
Initially, a second aspect of the present invention will be discussed where the device is configured as an optical detector which is sensitive to detect a single photon. The detector of the second aspect comprises first and second active layers separated by a first barrier layer, and detecting means for detecting a change in a characteristic of the first active layer, wherein the first active layer is a quantum well layer capable of supporting a two dimensional carrier gas and the second active layer comprises at least one quantum dot, the device further comprises means for separating a photo-excited electron-hole pair.
Preferably the means for separating an electron-hole pair will be provided by a means for applying an electric field normal to the active layers. However, the device may be fabricated such that the internal field of the device allows separation of photo-excited electron-hole pairs.
The device is capable of detecting a single photon. This is because optical illumination of the device leads to a change in the charge occupancy of the quantum dots and this, in turn induces a change in a transport or optical characteristic of the first active layer. The first active layer will have excess carriers provided by a doped barrier layer or a gate etc.
Absorption of a single photon by the device results in a change in the occupancy of a quantum dot by one carrier and this in turn induces a change in a transport or optical characteristic of the first active layer. A single photon incident on the device will photo-excite one electron-hole pair within the device. One of these photo-excited carriers is trapped by a quantum dot and induces a change in a characteristic of the first active layer. For simplicity, it will be assumed that the photo-excited hole is trapped within the quantum dot. However, it will be appreciated by those skilled in the art that the electron can be the photo-excited carrier which is trapped within the dot
The detector of the second aspect is configured to detect the presence of a single photon either by the size of the device, the total number of dots in the second active layer, the layer structure of the device or in the actual detection mechanism of the device.
Preferably, to reliably detect single photons irradiating the device, the number of active dots in the device is less than 100,000, more preferably less than 10,000. To avoid confusion, the term active dot is intended to mean a dot in the active area of the device which is capable of trapping carriers during normal operation of the device. Dots which cannot trap charge or which are outside the active area are not active dots. Also, the term active area is intended to mean the area of the device which is subjected to the separating means and which contributes to a change in the characteristic of the first active layer. In some cases, the field will be applied by a gate and a mesa will be etched to define the device boundaries, the active area here will be the area of overlap between the gate and the mesa. In this case the size of the gate is chosen so that the number of active dots in the device is less than 100,000, more preferably less than 10,000. The active area defined by the gate can be less than 10xe2x88x928m2.
In alternative examples, the size of the active region is limited by reducing the size of the conduction channel within the first active layer. This may be done by using a split gate, which defines a conduction channel in the first active layer. Here the defined conduction channel will define the active area and not the area of overlap between the gate and the mesa. Preferably, the length of the conduction channel will not be more than 100 xcexcm, more preferably not more than 10 xcexcm. Another method to limit the size of the conduction channel is by etching or damaging regions of the first active region
The detecting means can either measure a change in the transport characteristics of the first active layer or the optical characteristics of the layer. For example, a change in the optical characteristics due to a change in the carrier concentration or electric field across the layer could be detected. Changing the carrier density in the first active layer alters its absorption or emission spectrum. A change in the electric field across the first active layer will cause a change in the energy or intensity of its absorption or emission spectrum. Changes in the carrier concentration, mobility or field across the layer also manifest themselves in the transport characteristics of the layer.
Preferably, the means for detecting a change in a transport characteristic is configured to detect a change in a transport characteristic over a length of 100 xcexcm in the transport direction of the first active layer.
The photon detector can operate to detect photons over a wide range of wavelengths.
The number of photons absorbed in each of the barriers and active layers depends on the incident photon flux, the absorption coefficient of the layers and their thicknesses. While not wishing to be bound by any theory, it is believed that the following mechanisms contribute to photon detection by the device.
If the incident radiation has a photon energy higher than that of the barrier layers of the device, electron-hole pairs will be photo-excited in the barriers of the device, in addition to either of the active layers. The internal field of the device, or the applied field, disassociates the electron-hole pair and sweeps them in opposite directions, one type of carrier will be trapped by the dot and this will produce a change in the transport characteristic of the first active layer.
In the situation where the photon energy of the incident radiation is lower than the band gap of the barrier layers, but higher than that of the first active layer and also higher than that of the second active layer, electron-hole pairs will be excited in both the first active layer and the second active layer. Essentially no carriers will be excited in the barrier. Since the first active layer typically has a higher absorption co-efficient than the second active layer, most of the carriers will be excited within the first active layer. Due to the internal or applied electric field, one of the carriers can tunnel into the quantum dot leaving the other carrier in the first active layer. This will also cause a change in the characteristics of the first active layer.
If the photon energy of the incident radiation is less than Me band gap of the first active layer but larger than the band gap of the quantum dot layer, then electron-hole pairs are excited in the quantum dot layer. One of these carriers can tunnel out of the first active layer leaving the other carrier trapped within the quantum dot layer. This also causes a change in the characteristic of the first active layer.
The device can also detect illumination which has a photon energy lower than the band gap of the quantum dot, if the quantum dot is designed to contain excess charge. In this situation, a single photon can excite excess charge out of the dot by an intra-band transition. It is then swept away from the quantum dot by applied electric field or an internal electric field. This will also lead to a change in the characteristic of the first active layer.
The applicant does not wish to be bound by a particular theory or explanation. However, it is believed that characteristics of the first active layer can be affected by the trapped carriers through two main mechanisms. In the first mechanism, the excess carries in the first active layer have an opposing polarity to the carriers which are stored in the dots. In the second mechanism, the carriers in the first active layer and the carriers trapped in the dots are of the same type.
The first mechanism will be explained using holes as the stored carrier and a detecting mean consisting of a 2DEG. However, it will be apparent to a person skilled in the art either electrons or holes can be stored in the quantum dots depending on the layer thickness and composition, doping polarities and applied biases.
A quantum dot in the second active layer is illuminated with a beam of radiation. The structure is biased so that it is energetically favourable for the electron to tunnel through the barrier to a 2DEG in the quantum well layer, leaving behind a hole stored in the quantum dot. This effects a change in the conductivity of the 2DEG.
Without wishing to be bound by any particular theory or explanation, the applicant believes that the change in conductivity is predominantly due to the positive charge stored in the barrier which alters the band bending and hence persistently alters the conductivity of the 2DEG. Thus, depending on the actual configuration of the device, the conductivity of the 2DEG can either increase or decrease. It is also believed that where the stored carriers are electrons, the negative charge stored in the barrier will change the conductivity of the quantum well layer. In the case where the stored charges are electrons, the quantum well will support a two dimensional hole gas.
In the second mechanism, the carriers stored in the dots are of the same type as the carriers stored in the well. In the situation where the carriers in the quantum well are electrons, the stored carriers in the quantum dots are electrons. Carriers are preferably provided to the dots via a doped barrier situated on an opposing side of the second active layer to the first active layer. This layer may also be a modulation doped barrier layer, with an undoped spacer layer adjacent the second active layer.
The applicant does not wish to be bound by any theory or explanation. However, it is believed that the dots contain excess electrons prior to illumination. The charged dots act as scattering centres for the electrons in the 2DEG, which consequently has a relatively low mobility. After illumination, the number of excess electrons in the quantum dots is reduced and the number of electrons in the 2DEG increases. A decrease in the negative charge in the dots results in an increase in the 2DEG conductivity. Also, the increase in the carrier concentration of the 2DEG causes an increase in the conductivity of the 2DEG.
The inverse structure may also be realised where excess holes populate the quantum dots prior to illumination and the quantum well supports a two dimensional hole gas.
To detect the presence of charge in the first active layer, it is preferable if at least two ohmic contacts are provided to the first active layer. A voltage will be measured between these two contacts, or a current will be measured flowing between them. For an accurate reading, it is more preferable if a four-terminal voltage measurement is used.
The detection means preferably differentiates (with respect to time), the measured characteristic of the first active layer in order to detect small changes in the characteristics e.g. the conductivity or resistivity of the first active layer. The detection means may also comprise means for detecting a change in the characteristic of the first active layer with respect to time. More preferably, the differentiated signal is fed through a pulse counter to count the number of detected photons. This pulse counter is ideally provided with a discriminator which determines the minimum signal level which will be registered by the counter. This helps to discriminate the pulses due to photons from the noise on the signal.
Preferably, the device is configured so that electrons are supplied to the first active layer. This is preferably done by means of a doped second barrier layer which is preferably provided adjacent the first active layer. More preferably, this doped barrier layer is a modulation doped bier comprising a doped barrier layer and an undoped spacer which lies adjacent the first active layer.
Devices according to the second aspect of the present invention either with or without a doped barrier layer overlying the layer of quantum dots, can also be used where the stored carriers provided to the first active layer are holes.
It should be noted that in some cases, it is preferable to provide a thin growth smoothing layer, say for example, GaAs, prior to forming the dot layer to aid growth of the dot layer. Often, the growth smoothing layer will be provided between a barrier layer and the second active layer.
The field normal to the active layers can be generated by providing a front-gate overlying the structure. The front gate may be a metal front gate or a doped semiconductor gate. More preferably, a back-gate is also provided. The back-gate may be a metal back-gate or a doped semiconductor back-gate. It is preferable if the front-gate is semi-transparent to radiation with an energy close to that of the quantum dot band-gap. Semi transparent is taken to mean that the gate transmits at least about 50% of the radiation incident on the gate. The front and/or back gate may be a fill gate, alternatively one or both of them may be a split gate. A full gate is preferable when the detection of photons causes a decrease in the resistance of the first active layer, a split gate is used to define a conduction channel and is preferably used when illumination causes an increase in the resistance of the first active layer.
The active area of the fill gate is preferably no more than 10xe2x88x928 m2. The split gate preferably defines a conduction channel of no more than 108 xcexcm, more preferably no more than 10 xcexcm.
The means for applying electric fields may also comprise a p-type and n-type terminal located on opposite sides of the first and second active layers. In other words, the structure is sandwiched between doped p and n-type layers.
The detector preferably comprises an antireflection coating provided on the surface of the device which is to be illuminated.
The energy spectrum of the quantum dots is dependent on its size, shape and local environment. Hence, different quantum dots possess different ground state energies and different optical transition energies. The device may comprise quantum dots of different sizes which require radiation of different frequencies to resonantly excite an electron-hole pair.
A convenient method of forming a layer of quantum dots is by using the Stranski-Krastanow growth mode wherein a first layer is grown on a layer with a different lattice constant to the first layer. The first layer proceeds by three dimensional island growth and small quantum dots can be produced which typically have lateral dimensions of less than 50 nm. A preferable material system for producing this device uses the growth of InAs, InGaAs or InAlAs quantum dots with GaAs or (AlGa)As barriers.
The device may be formed such that the 2DEG layer is grown before the quantum dot layer. However, the ordering may be reversed i.e. the 2DEG layer formed overlying the dot layer. Other lattice mismatched systems can be used such as InGaN or AlGaN. Another possible system for producing the dots uses strained SiGe Heterostructures
The device may also conveniently be formed from silicon. Here, the dots would be formed from an amorphous layer of silicon which forms dots after annealing at 800xc2x0. It will also be appreciated by a man skilled in the art that germanium could also provide another possible material for fabricating the device.
Detection of single photons is also enhanced if the device further comprises an absorption layer. Such an absorption layer can be a relatively thick layer, for example greater than 100 mn, which forms a barrier layer to the quantum dots. Photons are absorbed within the absorbing layer, creating electron-hole pairs within the absorbing layer. An applied electric field, or internal electric field, within the absorbing layer separates the electron and holes which are swept in opposing directions by the field. One polarity of carrier is swept into the quantum dot layer. Generally, the absorption layer would be provided outside the active region of the device i.e. the absorption layer would not be placed in between the first or second active layers. Such a device will generally also comprise a semiconductor substrate.
The photon detector of the second aspect of the present invention is not limited to a device which has just a single layer of dots. Two or more layers of dots may be provided to trap charge to affect the conductivity of the first active layer. Alternatively, the detector may comprise a plurality of first and second layers separated by a barrier. This device can be thought of as a plurality of photon detectors arranged on top of one another.
A photon detector array may also be fabricated comprising a plurality of pixels, each pixel comprising a photon detector as previously hereinbefore described. Such a photon detector array may be provided with a grid of bit-lines and word-lines, wherein each pixel is addressable by applying an appropriate voltage to a word-line and/or a bit-line. Preferably, the bit-lines and word-lines are configured to apply a control signal to the means for separating a photon-excited electon-hole pair.
In a third aspect, the present invention provides a method of operating the detector of the second aspect of the present invention, the method comprising the steps of:
illuminating the device with a beam of radiation to excite at least one electron-hole pair such that at least one carrier becomes trapped in the second active layer;
detecting a change in the transport characteristics in the first active layer.
Preferably, a external field will be supplied to separate the electron-hole pairs.
The device of the first aspect of the present invention may also be configured as a memory device. The operation of the memory device is largely similar to that of the photon detector in that illumination of the device causes a change in the occupancy of the dots, which, in turn, affects a characteristic of a quantum well layer which can be measured. The memory device will be described with electrons as the excess carriers within the quantum well. However, it will be appreciated that the inverse structure can also be fabricated with holes as the excess carriers within the quantum well
The device can be largely configured as that of the single photon detector. However, the device will usually have a larger active area than that of the optical detector. Also, the number of dots in the memory device will usually be larger than that of the photon detector.
As mentioned for the photon detector, the memory is believed to operate using either a first mechanism or a second mechanism. In the first mechanism, as described for the photon detector, the carriers in the quantum well have an opposing polarity to the carriers stored in the quantum dots. As for the detector, the first mechanism will be described using holes as the stored carrier. For a xe2x80x98writexe2x80x99 operation, the device is illuminated with a beam of radiation with an energy close to that of the band-gap of the quantum dot. As for the photon detector, the structure is biased so that it is energetically favourable for the electron to tunnel through the barrier to a 2DEG in the quantum well layer, leaving behind a hole stored in the quantum dot. As previously described, this affects the change in the conductivity of the 2DEG.
For a xe2x80x98readxe2x80x99 operation, the device is again illuminated with energy of near the quantum dot band gap, but now with a weaker intensity. If two holes have already been stored in the dot during the write operation, another hole cannot be stored. Hence, there will be no further change in the conductivity of the 2DEG. If no holes are trapped in the dot, a change in the conductivity of the 2DEG will be detected To reset the device with this mechanism, a bias is applied across the device so that the energy of the electrons in the 2DEG lie above that of the conduction band level electrons in the quantum dot. Therefore, electrons are transferred into the quantum dot which can then combine with the holes. This will set the device on the electron-hole pairs relax back to the ground state.
The memory can also work using the second mechanism. Here, the carriers stored in the dots are of the same type of carrier stored in the well. As we have assumed that electrodes are stored in the quantum well, the stored carriers and the quantum dots will also be electrons. It is believed that the dots contain excess electrons prior to illumination. The charge dots act as scattering centres for the electrodes in the 2DEG which consequently is a relatively low mobility. After a write operation, the number of excess electrons in the quantum dot is reduced and the number of electrons in the 2DEG is increased. A decrease in the negative charge of the dots results in a decrease in the scattering and hence an increase in the 2DEG conductivity.
During the read operation, the dot is illuminated yet again. If the excess electrons in the dot have recombined due to a write operation, then no change in the conductivity of the 2DEG will be seen. However, if the excess electrons in the 2DEG have not recombined, then there will be a decrease in the conductivity as previously described.
For both read and write modes of the first mechanism, the conduction band edge of the 2DEG lies below the first confined conduction band level of the quantum dot. Thus, electrons transfer from the dot to the 2DEG, as the 2DEG is energetically more favourable. To reset the device, the bias across the device is changed so that the energy of an electron in the 2DEG lies above that of the conduction band level of electrons in the dot. Thus, electrons are transferred to the quantum dots which can then combine with the holes. This resets the device as the electron-hole pairs relax back to the ground state.
To produce a memory structure with a good retention time, it is preferable if there is a large confining potential for the trapped carriers. This is equally applicable to devices which are believed to operate by either the first or the second mechanism. The confining potential is largely dependent on the characteristics of the first barrier layer. For the avoidance of doubt as used hereinafter, a barrier layer is a layer with a larger band-gap than that of the active layers.
To produce a large confining potential, it is advantageous to maximise the carrier potential discontinuity between the quantum dot and the barrier layer. The xe2x80x98sizexe2x80x99 of the confining potential is dependent on both the potential height of the barrier layer and the width of the barrier layer itself. A large barrier is taken to mean a barrier which has a large carrier potential discontinuity with respect to the quantum dot layer and/or a barrier which is relatively wide.
For example, if the holes are stored in InAs quantum dots, a large trapping potential can be created by choosing AlAs as the barrier material both above and below the dots.
The electric field normal to the layers can be varied to modulate the band structure of the device.
More preferably, the first and second active layer are coupled layers. For the memory structure, the layer are preferably weakly coupled. The coupling between the layers needs to be sufficiently strong to allow tunnelling of some carriers from the second active layer to the first active layer. However, the coupling must also be sufficiently weak to suppress the tunnelling of carriers from the first active layer back to the second active layer.
The quantum well can be thought of as a sheet of charge located within the first active layer. The position of the quantum well within the first active layer, with respect to the adjacent layers of the first active layer is dependent on the band structure. In order to achieve sufficiently weak coupling between the quantum well and the plurality of dots, it is preferable if the separation between the quantum well and the dots is between 10 nm and 500 nm.
For example, if the plurality of quantum dots are InAs (or AlInAs) and the first active layer is InGaAs (or GaAs), the tunnel barrier layer could be AlAs or AlxGa1xe2x88x92xAs (or GaAs) with a width of between 10 nm and 500 nm. More preferably between 10 nm and 200 mm.
The above described memory device can only be reliably read once, as the read operation causes a carrier to be trapped in a quantum dot. However, the device of the present invention can be modified to function as a memory which can be read an infinite number of times by using two (or more) layers of quantum dots.
In a fourth aspect, the present invention provides a semiconductor device comprising first and second dot layers separated by a first barrier layer, each of the first and second dot layers comprising at least one quantum dot and there being at least one aligned double quantum dot provided by a quantum dot in the first dot layer being aligned with a quantum dot in the second dot layer, the device further comprising means for separating an electron hole pair in an aligned quantum dot and dipole detection means for detecting the presence of a dipole in at least one aligned double quantum dot.
The means for separating an electron hole pair preferably comprises means for applying an electric field normal to the first and second dot layers. As for the photon detector, separating means may also be provided by an internal field in the device due to band-gap engineering.
The means for detecting a dipole will preferably be provided by a first active layer comprising a two dimensional carrier gas located close enough to the at least aligned double quantum dot, such that the presence of a dipole in the quantum dot affects a characteristic of the two dimensional carrier gas. The dipole may affect a transport characteristic and/or an optical characteristic.
The applicant does not wish to be bound by any particular theory or explanation, however, it is believed that the memory operates in the following manner:
Generally, there will be a plurality of aligned quantum dots between the first and second dot layers, however, to aid explanation, the device will be described just with one aligned quantum dot. The quantum dot in the first layer will be called a storage dot. The quantum dot in the second layer will be called a sense dot.
During a write operation, the storage quantum dot is irradiated with radiation of a predetermined wavelength such that electron hole pairs are photo-excited within the storage quantum dot. Electrons are created in the conduction band and holes in the valence band. The electric field across the device is set so that electrons tunnel from the conduction band of the storage quantum dot and escape from the region of the two quantum dots, while the holes remain trapped by the storage quantum dot. The quantum dot is illuminated so as to fill the valence band level with holes, which corresponds to bit xe2x80x981xe2x80x99 being written to this dot.
In a read operation, a lower electric field is applied across the device; lower than that for the write operation and the device is again illuminated with radiation of a predetermined wavelength. If bit xe2x80x981xe2x80x99 has previously been written to this quantum dot, so that the appropriate valence band level is fully occupied with holes, the illumination is unable to excite electron-hole pairs within the quantum dot. Therefore, there is no variation in the charge state of either the storage quantum dot or the sense quantum dot.
On the other hand, if bit xe2x80x980xe2x80x99 has previously been written to the quantum dot, so that it is empty of holes, the incident light is able to excite electron-hole pairs within the quantum dot. The electric field is set so that holes are trapped within the storage quantum dot, while the photo-excited electrons are swept into the sense quantum dot. Since the electric field for reading is lower than the electric field for writing, the electrons are not swept out of the storage quantum dot, instead there are typed within the storage quantum dot. Thus, a dipole is formed between the electrons trapped in the sense quantum dot and the holes are trapped in the storage quantum dot This dipole is detected by the means described below.
Thus detection of an electrical dipole under optical illumination at the predetermined wavelength means that the dot was storage dot was empty of holes and corresponds to bit xe2x80x980xe2x80x99. On the other hand, if no dipole is detected under illumination by the predetermined wavelength, the dot must have been occupied by holes and corresponds to bit xe2x80x981xe2x80x99.
A dipole detection means, for example, a means for detecting a variation in the characteristics of a carrier gas in the vicinity of the first and second quantum dot layers, can be used to detect the presence of a dipole and hence can be used to detect whether or not the quantum dot has previously been written with bit 1.
After the read operation, the electric field across the dot layers can be reduced to allow the electrons trapped within the sense quantum dot and the holes trapped within the storage quantum dot to recombine, Also, the electrons trapped in the sense quantum dot will combine with holes in the storage quantum dot over time, after the illumination is switched off. It should be noted, because the electron and holes which are excited during the read operation are able to recombine with one another after the read operation, this will not overwrite the previously stored information.
For optimum operation, it is highly preferable that during the read or the write operations, carriers are not photo-excited in the sense quantum dot. Therefore, preferably, the band gap energy of the sense quantum dot is larger than the band gap energy of the storage quantum dot, so that the sense quantum dot is transparent to light with an energy equal to the band gap of the storage quantum dot. The sense quantum dot can be between the storage quantum dot and the active layer, it may also be located on an opposite side of the storage quantum dot to the active layer.
This variation in the band gap between the quantum dots of the first dot layer and the second dot layer can be achieved in a number of ways. For example, the first and second dot layers can be made out of different materials or materials with different alloy compositions. Also, the strain environment of the dots in the first and second layers could also be varied to provide a difference in the band gap of the quantum dots. Preferably, the first dot layer forms a type I heterojunction with its barrier materials and the second dot forms a type II heterojunction with its barrier material.
The detecting means can either measure a change in the transport characteristics of the active layer or the optical characteristics of the active layer. For example, a change in the optical characteristics due to a change in the carrier concentration or electric field across the layer induced by a dipole in an aligned double quantum dot could be detected. Changing the carrier density in the active layer alters its absorption or emission spectrum. A change in the electric field across the active layer will cause a change in the energy or intensity of its absorption or emission spectrum. Changes in the carrier concentration, mobility or field across the layer also manifest themselves in the transport characteristics of the layer.
The configuration of the barrier layers in the device with an aligned quantum dot, is generally different to that described above for the single layer of dots. The terms xe2x80x98first barrier layerxe2x80x99 etc have been re-defined below for the device with the aligned quantum dot. The definitions of the barrier layers below only refer to the device with the aligned quantum dot and do not refer to the single dot layer structure.
The first barrier layer is a barrier layer located between the two dot layers. The first barrier layer will generally have a larger band gap than the first dot layer and the second dot layer.
Preferably, there will also be a second barrier layer located between the active layer and the closest of the dot layers to the active layer. However, this is not always required. For example, in certain device configurations, both the first active layer and the second xe2x80x9cbarrier layerxe2x80x9d may be the same material e.g. GaAs. The GaAs barrier layer is an effective barrier layer even though it has the same band gap as the active layer.
There may also be provided a third barrier layer, which is located on an opposite side of the first dot layer to the second dot layer. This third barrier layer will have a larger band gap Man the first dot layer. It may also have a larger band gap than the first barrier layer. The third barrier layer prevents trapped carriers tunnelling out of the first dot layer. The third barrier layer may be doped to provide excess charge for the first dot layer.
Preferably, excess carriers are supplied to the active layer from a fourth doped barrier layer. More preferably, the fourth barrier layer is provided on an opposing side of the active layer to the dot layers. The fourth barrier layer may preferably be a modulation doped barrier layer comprising an undoped spacer layer adjacent the active layer and a doped layer adjacent the undoped spacer layer.
Barrier layers may be located adjacent to either or both of the first and/or second dot layers. However, in some cases, a thin layer may be formed between a barrier layer and a dot layer to aid smooth growth of the dots during fabrication. Such a thin layer will be referred to s a growth smoothing layer. Typically, if the dots are formed by depositing InAs or InGaAs, the layer may be GaAs.
To produce a memory structure with a good retention time, it is preferable if there is a large confining potential for the trapped carriers. The confining potential is largely dependent on the characteristics of the second and third barrier layer.
To produce a large confining potential, it is advantageous to maximise the carrier potential discontinuity between the dot layers and the barrier layers. The xe2x80x98sizexe2x80x99 of the confining potential is dependent on both the potential height of the barrier layer and the width of the barrier layer itself. A large barrier is taken to mean a barrier which has a large carrier potential discontinuity with respect to the quantum dot layer and/or a barrier which is relatively wide.
For example, if the holes are stored in InAs quantum dots, a large trapping potential can be created by choosing AlAs or AlxGa1xe2x88x92xAs as the barrier material both above, below and between the dot layers.
The electric field normal to the layers can be varied to control the tunnelling of photo excited electrons and holes through the structure.
The first barrier layer can be fairly thin, for example less than 30 nm. The width of the first barrier is generally determined by the limits of the fabrication technology available to produce InAs aligned quantum dots, at present, good aligned quantum dots can be produced with a first barrier width of about 15 nm.
More preferably, the active layer and the second dot layer are coupled layers, preferably, weakly coupled layers. The coupling between the layers needs to be sufficiently strong to allow tunnelling of some carriers from the dot layer closest to the active layer. However, the coupling must also be sufficiently weak to suppress the tunneling of carriers from the active layer back to the dot layers.
The quantum well can be thought of as a sheet of charge located within the active layer. The position of the quantum well within the active layer, with respect to the adjacent layers of the active layer is dependent on the band structure In order to achieve sufficiently weak coupling between the quantum well and the second dot layer, it is preferable if the separation between the quantum well and the second dot layer is between 10 nm and 500 nm.
For example, if the plurality of quantum dots in for the second dot layer are InAs (or AlInAs) and the active InGaAs (or GaAs), the second barrier layer could be AlAs or AlxGa1xe2x88x92xAs or GaAs with a width of between 10 nm and 500 nm. More preferably between 10 nm and 200 nm.
To detect the presence of a dipole formed in an aligned double quantum dot, it is preferable if at least two ohmic contacts are provided to the active layer. A voltage will be measured between these two contacts or a current flowing between the two contacts. For an accurate reading, it is more preferable if a four-terminal voltage measurement is used.
Preferably, the measured characteristic from the active layer will be differentiated with respect to time or the change in the characteristic will be measured with respect to time. When the signal is differentiated, the differentiated signal comprises a plurality of pulses, these pulses can be counted by a pulse counter.
The memory device with the aligned double quantum dot either with or without a doped barrier layer overlying the first dot layer, can also be used where the tunnelling carriers are holes.
If the electron hole pair is separated by an externally applied electric field normal to the dot layers, this can be provided by a front-gate overlying the structure. More preferably, a back-gate is also provided in addition to or instead of the front gate. Either or both of the front or back gates can be metal or a doped semiconductor. It is preferable if the front-gate is transparent to radiation with an energy close to that of the quantum dot band-gap of the first dot layer.
The means for applying electric fields may also comprise a p-type terminal and n-type terminal located on opposite sides of the first and second dot layers. In other words, the structure is sandwiched between doped p and n-type layers.
The energy spectrum of the quantum dots is dependent on the dot size, shape and local environment. Hence, different quantum dots possess different ground state energies and different optical tuition energies. Preferably, as mentioned previously, there may be a difference between the size of the dots in the first and second dot layers. The device may also comprise quantum dots of different sizes within each layer which require radiation of different frequencies to excite an electron-hole pair.
A convenient method of forming a layer of quantum dots is by using the Stranski-Krastanow growth mode as described for the photon detector of the second aspect of the present invention. The Stranski-Krastanow growth mode can also be used to produce aligned double quantum dots. This involves depositing a first strained layer which self-assembles into a layer of quantum dots. This is followed by a first barrier layer. Then a second strained layer is deposited which self-assembles into a second layer of quantum dots. It has been found that if the first barrier layer is thinner than a cent thickness, the quantum dots in the second quantum dot layer spatially align above the dots in the first. This is thought to be because the strain created by the dots in the first quantum dot layer seeds the growth of quantum dots in the second quantum dot layer. For the growth of InAs dots on AlxGa1xe2x88x92xAs, for instance, the second barrier layer should be no more than about 15 nm thick. Conveniently, it has been found that the dots in the second quantum dot layer have a larger band gap energy than those in the first.
The above description of growth has used the terms xe2x80x9cfirst quantum dotxe2x80x9d layer etc. The terms xe2x80x9cfirstxe2x80x9d and xe2x80x9csecondxe2x80x9d in the above paragraph refer to the layers in order of growth and are not intended to suggest that the storage dot layer must be grown before the sense dot layer. The dot layers may be fabricated in any order. However, due to the dots in the second grown layer having a larger hand gap than the dots in the first grown layer, if the dots in the fist and second dot layers are fabricated from the same material, the dots in the second grown dot layer lend themselves more easily to sense dots which preferably require a larger band gap.
The device may also be fabricated from an SiO2/Si based system.
The above described memory devices, i.e. both the device with the single dot layer and the aligned dot can also be used as a holographic type of optical storage device. Here, the optical beam is split into a signal beam and a reference beam. The signal beam is passed through a spatial light modulator in order to encode the information to be stored. The signal and reference beams are focused onto the surface of the sample where they produce an interference pattern. This creates a spatial variation in the carrier occupancy of the dots which acts to store information in the dots. The information can be recalled by illuminating the same area of the sample by the reference beam. The stored variation in the dot occupancy acts to diffract the reference beam and thereby to recover the signal beam which is detected by a suitable means, such as a charge coupled device array.
The memory structures may be provided with upper and lower cladding layers to channel light in a direction parallel to the plane of the first active layer. The structure may also be provided with guide means to confine light to a region of the active layer, for example, a stripe type waveguide could be used.
The memory structures may also be illuminated in the plane of the dot layers instead of or in addition to illuminating generally perpendicular to the layers.
The device of the fourth aspect could be operated by:
applying a field across the dot layers sufficient to allow carriers of a predetermined polarity to tunnel from the first dot layer to the second dot layer under illumination;
selectively illuminating one or more of the plurality of quantum dots with a beam of radiation to excite at least one electron-hole pair such that at least one carrier can tunnel from the first dot layer to the second dot layer; and
detecting the presence of a dipole in an aligned double quantum dot.
The above method is primarily intended as a xe2x80x9creadxe2x80x9d operation.
More preferably, the method also comprises a write operation which can be performed before or even after a read operation.
Therefore, it is preferable if the method also comprises a step of changing the electric field across the device such that the field can be changed between a field configured to allow a carrier to tunnel from the first dot layer to the second dot layer wherein the carrier becomes trapped in the second dot layer and a field configured to allow a carrier to tunnel from the first dot layer such that the carrier does not become trapped in the second dot layer.
For a xe2x80x9cwritexe2x80x9d operation the carrier does not become trapped in the second dot layer.
The device has been described with just two dot layers, a sense dot layer and a storage dot layer. However, the device could be fabricated with a plurality of dot layers such that one layer acts as a storage dot layer and one or more of the plurality of dot layers acts as a sense dot layer. In some cases, this might be useful to enhance confinement between the storage dot layer and the sense dot layer during the read operation.
The device may also comprise a plurality of first and second dot layers separated by a tunnel layer and means for each of the plurality of first and second dot layers of detecting a dipole between the first and second dot layers.
The present invention is primarily intended for use as an optical memory. Therefore, preferably, an optical memory is provided comprising a plurality of pixels wherein each pixel comprises a device according to the fourth aspect of the present invention or the device configured as a memory according to the first aspect of the present invention.
Each device can be addressed by applying a voltage to a word-line or a bit-line, or both, in the conventional manner. However, the present invention provides yet another dimension to the memory as each pixel can comprise a plurality of quantum dots each of which have a different predetermined excitation energy. Therefore, not only is it possible to address each pixel by varying the voltages on the bit-line and the word-lines, it is also possible to address a single quantum dot by illuminating a pixel with a specific wavelength. Preferably a monochromatic light source should be used for this excitation such as a laser diode. Each of these pixels may contain one or more switching transistors for addressing of the appropriate device.
The bit-line and word-line voltage can be applied directly to the semiconductor device to provide the field for either reading or writing to the quantum dot The array of pixels can be illuminated by a relatively broad beam of light, which can illuminate more than one, or all, of the pixels simultaneously. However, in this case only the active pixel i.e. the pixel with the correct word and bit-line voltages applied to it will store or provide data Also, if the wavelength of the input beam can be varied then another dimension of storage is provided since a plurality of dots may be provided with different band gaps and hence different excitation energies.
Therefore, it is preferable if the optical device of the fourth aspect of the present invention used in the pixel comprises a plurality of aligned double quantum dots, wherein at least one aligned quantum dot has a different excitation energy to that of the other aligned quantum dots.
The memory devices describes above are a subset of an addressable optically active memory structure. Therefore, in a fifth aspect, the present invention provides a memory structure comprising a grid of bit-lines and word-lines defining a plurality of pixels, each pixel being addressable by applying potentials to the bit-line or word-line, at least one pixel comprising a device having a plurality of optical storage means, wherein at least one optical storage means capable of storing data due to optical activation at a different wavelength to that of at least one other optical storage means in the same pixel.
Therefore, the memory according to a fifth aspect of the present invention is addressable via two voltages and a wavelength A method of operating such a device could comprise the steps of applying one or both of the voltages to select the pixel and illuminating with light with a wavelength which can excite a transition in the required memory element of the pixel.
The terms bit-lines and word lines have been used as these are well understood in the an to mean a grid of wires or tracks, which can be used to address a pixel.
The device having optical storage means may be provided by any optically activated memory structure. The above described memory structure with either a single dot layer or a layer of aligned can be devices having the plurality of optical storage means. The individual storage means being a single quantum dot or an aligned quantum dot etc.
Alternatively, the device may be addressed by a focused light source which can be scanned over the surface of the device into different pixels. Preferably this is a monochromatic light source such as a laser diode. Preferably the light is focused to a spot with a diameter of 10 microns or less. In this case it is possible to deflect the focused laser spot across the surface of the device using a pair of mirrors mounted on galvanometers.
Therefore, in a sixth aspect, the present invention provides a method of operating a device comprising a grid of bit-lines and word-lines addressing a plurality of pixels, each pixel being addressable by applying appropriate potentials to the bit-line and/or the word-line, at least one pixel comprising a device having a plurality of optical storage means, wherein at least one optical storage means is capable of storing data due to an optical activation at a different wavelength than that of at least one other optical storage means, the method comprising the steps of:
setting the word-line and/or the bit line voltage to switch a pixel into a read mode or a write mode;
illuminating at least the said pixel which is in a read or write mode, with a beam of radiation with a predetermined wavelength which can activate at least one of the optical storage means in the said pixel.
In the above method, the step of illuminating the pixel can comprise the step of illuminating a plurality of pixels with a broad beam, or scanning a narrow beam, which can only illuminate one pixel at a time to the said pixel.
The step of scanning the read/write beam could be performed by moving the beam via moving mirrors. The mirrors could be controlled by galvanometers.
The device according to a first aspect of the present invention can also be configured as an amplifying photo-transistor. The change in the conductivity of the 2DEG can be used to detect the absorption of light After the photon is absorbed, the photon-excited electron and hole are spatially separated into the well and dot layers creating a detectable change in the conductivity of the 2DEG. The mobility of carriers in the first active layer can be modulated by changing the electric field across the first active layer. The mobility can be optimised such that when a carrier tunnels from a dot to the first active layer, a larger rise in conductivity of the device is seen than if the vertical photo-current was measured directly.
A high speed MODFET can also be realised using this structure. Here, the total carrier density in both the well and the dot is maintained constant. When the device is biased so that all the electrons are in the well, the source-drain conductivity is high. However, when the device is biased so that some of the electrons are transferred to the dots, these electrons are trapped and cannot contribute to the conductivity of the device. Therefore, the conductivity is reduced. The electrons tapped in the dots also contribute to scattering of the carriers in the 2DEG, fiber reducing the mobility of the 2DEG.
The switching speed of this device is limited by the tunnelling time between the dots and the well. For fast switching response a smaller barrier should be used, the can be either a thin barrier or a barrier with a relatively small band-gap for a barrier layer. Typically, for example, the barrier could be between 0.5 nm to 5 nm of AlGaAs.
The MODFET can potentially be switched with frequencies of about 100 GHz to 10 THz.
A barrier layer is required otherwise a large separation between the first and second active layers is required to allow resonance of the electron levels at achievable electric fields.
The MODFET could be operated by a method comprising the steps of:
measuring the resistivity of the first active layer via the ohmic contacts; and
changing the resistivity of the first active layer by transferring carriers between the first and second active layers by altering the field across the active layers.
The absorption of incident radiation is dependent on the population of the quantum dots.
Therefore, this device can be configured to work as an optical switch or an electro-optical modulator. When the dots have excess carriers, optical absorption is suppressed increasing the optical transparency of the device. In a zero dimensional system there is a large energy separation between the ground and excited states. Therefore, just adding two electrons or holes to the quantum dot completely suppresses absorption of radiation with an energy close to that of the quantum dot band gap.
The optical modulator will be practically realised by incorporating the quantum well Structure into a waveguide for channelling the light along the plane of the layers. Preferably, upper and lower cladding layers are provided, wherein the first active layer overlies the lower cladding layer and the upper cladding layer overlies the second active layer. Again, this device can be operated at a very high frequencies ( greater than 100 GHz).
It may also be preferable to provide guide means for confining light to a predetermined region of the first active layer. For example, a strip-type waveguide layer may be provided overlying the structure.
For the MODFET or optical modulator structures, it is preferable if the first and second active layers are more strongly coupled than for the memory structure to allow tunnelling in both directions between the first and second layers. The smaller separation between the dot layer and the 2DEG allows more effective coupling of the dots to the 2DEG and hence, faster and more efficient tunnelling.